1. Field of Art;
The present invention relates generally to a casting system which is particularly suited for the mass production of cast articles. More particularly, it relates to a system for sintering molds and for molding cast products in a continuous sequence, the molding steps including the step of casting a molten metal material into the sintered molds and the step of cooling the cast material contained in the molds. In the system of the invention, the cooling step is divided into two-stage operations including an initial rapid cooling operation and a subsequent slow cooling operation effected at a controlled rate. The system of the invention affords eminent advantages when used for the production of cast articles made of a material susceptible to cracking due to improper cooling.
Typical examples of such material are casting grade magnesium alloys which have generally wide temperature range within which they are in the solid solution phase. Such temperature range will be referred to as "solid solution forming temperature range" throughout the following description and in the appended claims. Furthermore, cast magnesium alloys immediately after solidification thereof have inferior strength to those of other cast alloys including aluminum alloys. For these reasons, the cooling rate for solidifying a magnesium alloy should be strictly controlled in order to prevent increase in internal stress, due to shrinkage upon solidification, which might cause cracking at a portion having a thin or uneven wall thickness, such a cracking being induced by the interaction between the shrinking cast alloy and the mold containing the same.
2. Prior Art;
In the conventional technology, it has been a common practice to control the cooling rate for suppressing extreme internal stress which often induces adverse cracking, by cooling the cast alloy slowly in an annealing furnace over a sufficient time period. However, in consideration of the efficiency and the running cost of the entire system, it is desirous that the time required for the overall cooling should be decreased as short as possible. In this connection, the known prior art system is not fully satisfactory, since a long cooling time is necessitated to lower the efficiency thereof.
On the other hand, if the cooling time is simply decreased in the prior art system, extreme difficulties are encountered to produce faultless cast articles at high yield due to the aforementioned cracking problem.
Another disadvantage of the prior art system resides in that the sintering furnace for sintering therein molds and the annealing furnace for slowly cooling the cast alloy or metal contained in the molds, both having considerably large dimensions particularly being long in lengthwise dimension to require vast areas, are built independently at separate locations. With such arrangement, a large area is necessary for the entire system installation. In addition, piping and other concomitant fitting members are indispensable for combining the sintering furnace with the annealing furnace, leading to increase in investment.
Furthermore, the down time of any one or two of the sintering furnace, the casting device and the annealing furnace is often increased because of the difficulty in well timing the operation time throughout the system.